1. Field of the Invention
This invention relates to the field of polymeric compositions suitable for use in forming contact lenses. More particularly, the invention concerns a disposable, thin membrane formed for ocular and other therapeutic use from the aforementioned composition, which membranes can be worn for long periods of time without discomfort; the method of making such a membrane, and the membrane itself.
2. Description of the Prior Art
Optical contact lenses are conventionally made of either hard plastic, such as, acrylic resins or materials which are hydrophilic and water swellable (the so-called "soft lens"). The resins utlized conventionally must, of course, possess the various properties required for the ophthalmic use with respect to their light transmission, durability, etc. However, all of the known lenses present the basic defect that they cannot be worn regularly in the eye for a long periods of time, i.e. for more than about fourteen to eighteen hours. Most certainly, their use is not recommended during sleep. Consequently, the contact lenses known to date should be inserted and removed daily from the eye.
Furthermore, those "hard" lenses made from the polymethylmethacrylate resins possess excellent rigidity and durability for handling by the user. However, they are highly impermeable to oxygen and carbon dioxide which is necessary for the proper metabolic functioning of the eye. These resins are also hydrophobic and as a result lack surface wettability.
The so-called "soft" lenses, which are water swellable and hydrophilic must be maintained under conditions of sterility. This results in various inconveniences to the user.
Moreover, the "soft lenses" which are made from water swellable, hydrophilic polymers do not have the durability of the "hard" acrylic resin materials and are much more difficult to handle. Further, they are water swellable and can absorb an amount of water which is equal to many times the original volume of the lens. Consequently, the dimensions of such lenses are constantly undergoing change, which, in turn, changes the refractive index, and the refractive or ophthalmic properties of the lens.
Also, when such lenses are manufactured in the dry, unswollen state, the ultimate dimensions of the lens can vary greatly because minor deviations from standards will be amplified many times when the lens fully swells. Therefore, the "soft" lenses are relatively difficult to manufacture to the exacting tolerances required, as is repetitive, production or duplication of lenses of the same refraction and curvative parameters.
An additional problem with such contact lenses arises due to the fact that the cornea, on which the lens rests or covers, must have an adequate supply of oxygen, much of which is supplied by the tear fluid which bathes the cornea. This, oxygen, presumably from the atmosphere, dissolves in the tear fluid and it is this dissolved oxygen which is then transferred to the cornea.
The so-called "soft lenses" like their "hard" counterparts, do not possess satisfactory permeability to oxygen and consequently offer no solution to this problem.
Attempts have been made to use a polysiloxane gum as a contact lens because of its high permeability to oxygen. See, for example U.S. Pat. No. 3,228,741. However, this particular material is quite hydrophobic. In order to assure proper comfort and wearability in a contact lens or material that is in direct contact with the eye, it is important that the material be wettable, i.e., so that the tear fluid will form a continuous film on the lens and not bead up. In use, this film is renewed at each eyelid blink so that, as a practical matter, the material must have sufficient wettability to allow formation of a continuous film of tear liquid which film will last for the duration of the blink period.
The cross linked polysiloxanes do not possess this property and have thus found little use in the contact lens field.
Another problem with the siloxane materials is that, when placed in the eye, the surfaces of the lens attract ocular debris, e.g., proteins, lipids, mucoids, and the like, which the eye is constantly giving off. One of the functions of the tear liquid is to wash away this debris. However, the polysiloxane material is lipophilic and mucophilic, i.e., attracts this ocular debris, and this further restricts the siloxane material of opthamalmic use.
Attempts have also been made to modify such polysiloxanes by grafting, i.e., coating the surface thereof with another type of polymer. Such attmepts have been generally unsuccessful because of the difficult techniques involved in obtaining lenses and optically acceptable membranes from such nonuniform systems.
In my related applicaton Ser. No. 677,246, I disclosed a new polysiloxane copolymer composition which possesses a number of properties which makes it highly suitable for use as an ocular lens, among which are excellent wettability and oxygen permeability.
Additionally, the composition disclosed herein allows for the accurate reproduction of tolerances and lens parameters.